Outdoor Reset a myth?

S. Orbine

I've got a boiler / air handler with lots of ducting in the uninsulated attic space. The whole system is new, but the ducts lose about 15% of their heat mostly due to poor insulation. (New construction is just crap) Anyway, the outdoor reset sets the water temperature lower on warmer days, resulting in longer run time and thereby more heat loss through the lousy duct system. Even good ducts would lose, say 5% through conduction and more through leakage.

Question is, it seems the system is more efficient if I set the boiler temperature high (say 200), and have the system run as little as possible. This minimizes duct loss.

Where is the efficiency of outdoor reset supposed to be gained?

Thanks for your input!

hydronicsmike

what??

In my opinion, the losses are always greater when the difference between surface temperature (duct) and ambient air temperature is greater. That works for anything. So when the air temperatrue within the duct is lower (due to Reset) the losses of your Duct are (should be!!) also lower, aren't they?

Since the losses are greater at higher temperatures, it takes more energy to recover the loss. Does this make sense?

Boilers are not 90-95% efficient at 200°F. At 140°F maybe. Hence the Reset. A system is never designed to be as efficient as possible for the worst time of the year when 100% capacity is required. But the system will run as efficiently as possible for the majority of the year while operated at lower temperatures.

Is insulation a myth too?? As I understand it, most people concerned about energy savings and usage use insulation.


Regards,

Mike

Ron Schroeder_4

If your losses are due to leakage from the ducts into the attic, you might be right but if it is just due to poor insulation on the ducts, reset will help you.

Ron

Rich W

loss

I agree with Mike. It's like having a car that gets 50 miles per gallon. It doesn't do you much good if you have a hole in the gas tank...your ductwork should be sealed AND insulated.

bob_44

Efficiency

may be the least of your problems. If you are losing heat into your attic you will probably have trouble with ice dams and water damage. Seal that duct as air tight as is humanly possible and insulate the heck out of it. Make sure your attic is well ventilated.

don_65

The myth

is that most people don't believe that there ducts need to be seal.Mike in all the other are correct.

You must look at your system has a whole and not focus on just the reset.

Duct leakage amplify the envelope infiltration loads and
disrupt the BALANCE of the air distribution system.

Conduction losses are a big factor as well,dont focus only on one thing.

Rely_2

Door test

Steve I have put in many systems that are high eff boiler with hydronic coil in ductwork we always seal and insulate.New Jersey reqires a door test on the system and the allowance is 10% of the air flow on A/C fan.We with the sealed duct have had test results come back with 0 to 1 % loss on system.Burnham heating helper states that nosealed duct will give a loss of 20 %.Seal the duct and use 2" fsk insulation

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S. Orbine

boiler efficiency

Mike,

Thanks for the reply. You've tickled my curiousity about boiler efficiency vs. water temperature. I have two Burnham RV5 (Revolution) boilers, but I've never been able to find any data on efficiency vs. temperature, and I have looked ! Where should I be looking?

Also, is that the key benefit to outdoor reset, increased efficiency at the boiler?

Thanks,

Steve

S. Orbine

Hello,

Well first, I don't know about a door test here in NJ. I think that may be only for 'Energy Star'. I can tell you there was no such requirement for us. In fact, things like ACCA manual D (or J) are not spoken of in central NJ! I've come to believe that those who apply them are very few and far between.

Regarding the insulation, there's a combination of flex and hard duct:

Flex: mostly R6. I've been beating my brain about how I would add effective insulation.

Hard: is covered with a fiberglass/foil product marked R6, but I would need to remove all of this first, seal the joints properly, and reinsulate. Again, hard to do with batts from Home Depot! The present insualtion is wrapped tightly (compressed) which appears to be a problem.

S. Orbine

OK, OK, I agree with all of you. The ducts do have to be sealed and insulated. I will most likely remove all the ducts and reinstall. I think in the end it's the easiest way to fix them. I'm past that in my mind.

The question is: Where, theoretically, does outdoor reset contribute to efficiency? Mike suggested the boiler is more efficient at low temps??

I've run a bunch of numbers, and while loss / hr is lower at lower temperature, the number of hours is greater and the net loss is higher.

Back to the ducts for a second, I ran a 30-minute (to stabilize everything) test with the air handler running, but no hot water to the coil. 1100 cfm, air in (at register) 71 degrees, air out (at register) 55 degrees. Attic temp 30F. With surface area of ducts, cfm, etc the effective duct insulation is R2.5 So there is an insulation problem, but in the meantime I'm wondering how the heck is outdoor reset supposed to save energy? Even at R10 on the ducts, the cooler temps do not make up for the longer run times.

Steve_35

Look for a company that does

AirSeal. It's a product that will seal holes up to 5/8" in the duct work from the inside. It's a pretty amazing product. I believe it's guaranteed to keep them sealed for 10 years.

And you may be right on about the longer run times not saving you much, if any money. You've also got to factor in the juice your blower motors are using and if they're not ECM motors it's significant. IMO, if your ducts are properly sealed and insulated the electrical usage can easily be more significant than the fuel savings. I see this when I do an energy analysis with WrightSoft comparing a straight 90% furnace vs a 90% two stage. We get longer run times with the two stage but the fuel savings are almost offset by the increased electrical usage unless it's a variable speed blower. This is one the great advantages of jull hydronic systems. It takes very little power to move 30-40,000 btuh with water.

The biggest advantage with restet is comfort. If we can move the exact number of btus into your home that it is losing you will be the most comfortable. Particularly if you have a variable speed blower. A problem with forced air is the stratification. But keep the air moving and that is much less of a problem.

Constantin

they do and they don't.

Outdoor reset is (OR) more than cruise control. It anticipates heat loss as a function outdoor temperature and the curve that was programmed into the unit.

OR has it's application primarily on the water-side of the business because radiant systsms and most hydronic systems have a lot of mass. Since heating systems operate on a closed-loop cycle, there is usually no way to remove excessivly hot water from under a slab, a radiator, etc. and replace it with colder water.

The high heat potential of water will then make the rooms too hot, causing occupants to open the double-hung dampers (i.e. the windows) to bring the temperature back down to reasonable levels. OR helps by better controlling the heating system to prevent under- and over-shoots. However, even OR has it's limits, as taming a radiant-heated room with the potential for a sudden high solar gain (think window walls with a southern exposure) is pretty tricky.

Air-based systems are likely to benefit less from OR simply because there is not much mass in the system. Air is a terrible heat conductor and its capacity to carry heat is bad as well. However, the big benefit of a reset-controlled boiler / hydro-air combo is that you can dial in the perfect coil temperature to keep the place at an even temperature even with a single-speed blower. Most variable-speed air handlers try to replicate some of that control by varying their airspeed, for example, but that's expensive.

Plus, the lower the duct temperature/attic temperature differential, the less heat loss is going to occur via the ducts into your attic. Since most of our heating is in the shoulder months, not design days, there is a 13-26% energy conservation potential due to OR in most applications.

I'd also investigate aeroseal.com as an alternative to taking the ducts down and starting over. Another option is to fill the rafter bays with insulation and making the attic a part of the conditioned space. If you're a fan of cold roof venting, you can always put some vents behind the fiberglass,etc. However, I'm a fan of foam-insulated roofs, and chose a exterior shingle that can take the extra heat.

In closing, OR will save you some money, regardless of the heating system you have in the home. However, the degree to which it benefits your home and wallet is also a function of the heating system. For your system, I would implement OR simply on the basis of comfort, and take the savings as a bonus.

EBEBRATT-Ed_2

If you think of the air system as a closed hydronic loop eaven though it is an open system you can see what is happening. If a hydronic system leaked you would be adding cold make-up water. The air system is no different. If you have duct leakage in the supply ducts and are losing air into the attic this air must be made up by outside air leaking into the house (infiltration). The ducts must be sealed and insulated.

As far as reset controls I always thought that the boilers and furnaces must be hot to get maximum Co2 and efficiency. Is here any information on part load and efficiency at lower water/air temperatures?? I agree that system losses from pipes and ducts are less at lower water and air temps.

Sweet_2

pitman9

Im certainly not trying to turning this into HVAC forum but those D.C. motors there using I think Emerson is the manufacturer, Ive gotten alot of complaints about the ramp up and down that comes with the variable motors Sounds like a low groan noise have you experienced this at all?

joel_23

The Big Picture

Reset works there is no question about it. I won't go into that and I also won't cover the duct seal/insulate thing. I'll move on to something not covered yet by anybody that is perhaps the most critically important point a revelation that you may not have given enough thought to. Ready?.... your heating systems is installed OUTSIDE your home. The attic is effectivly the same temperature as the outside. This of course is a very bad place to run ductwork or piping. You noted how hard it would be to try and reinsulate the flex duct to higher levels. Your right so instead get a Icynene contractor to spray foam the ROOF of your house effectivly making the attic space conditioned space. How effective is this? Well last year we did a remodle on a house with the unit in a non conditioned attic. During the remodel the sqft of the house was doubled and the glass area was almost trippled. The house was completely Icynened including the roof rafters moving the attic system from OUTSIDE to INSIDE the envelope. The result? despite doubling the size of the house and trippling the glass , the same size A/c was used. The owners report Better comfort than before and lower bills! Disclaimer: I'm not associated with Icynene in anyway I'm an HVAC contractor. While your on the the right path with the ducts we really should be examining in more detail how we biuld and insulate the house itself. IMHO it should be illegal to put ducts/pipes in unconditioned spaces, meaning outside the envelope.

Constantin

Umm, joel...

... Didn't I mention rafter insulation further up? :-)

Anyway, I totally agree that the benefits of proper insulation in the attic far outweighs the cost of doing so.

The only downside that I am aware of is that cheap asphalt roof shingles allegedly have a tendency to degenerate faster when they longer have a "cold" vented roof on the other side. IIRC, buildingscience.com made some measurements in Las Vegas during the summer and showed a 14-20°F temperature differential between roof decks with Icynene on one side and ventilated roofs.

In most climates outside of the sun belt, I doubt that insulated, unvented roofs will have a significantly shorter life. I chose Icynene and Corbond for my rafter sections, as I would like to keep the mechanical space in the attic happy and don't think that my Duraslate will be affected one iota by slightly higher deck temperatures. Without the vents in the attic area, another significant infiltration source/exit has been plugged.

S. Orbine

Jpel,

You're absolutely right. The attic is no place for ducts, and it's really the root of the problem. All the thinking about it got me curious about the OR, and how is it supposed to work. Still don't know.

On those ducts. I haven't even figured a way to calculate loss from convection currents when the air handler is off. For sure that cold air is dropping out of those ceiling registers.

The sad part is that it cost me $47K for this headache. If I wanted a headache, a bottle of Seagrams would have been alot cheaper.

joel_23

Constantine

Yes you did mention it as part of a larger and well written post. However it did seem to me to get lost in the middle of the other stuff you where writing. I believe it of upmost importance and wanted to eleborate on it further.
But for the record everybody Constantine beat me to the punch!

Bill Nye_2

I think

the issue is COMFORT! I have hydro air in my home {I no longer use it} and when the boiler was up near high limit,180°, the air from the duct was uncomfortable. The room heated up rapidly but the air was "stuffy" and the temperature swings felt exaggerated. Cool/hot/cool/hot etc.

The air that was delivered with 140° water seemed more tolerable. I have radiant baseboard heat now. I have outdoor reset for the boiler water temperature. My room temperature never varies more than 1°F. With the hydro air I think the swing was 4or5° between cyles.

I was told that for every 3°F you turn back the water temperature of the boiler you could save 1% of your annual heating bill. I am not a scientist [that's my cousin] so I can not prove this formula, it is something I was taught years ago.

The higher air delivery temp. could lead to more stacking or stratification causing more infiltration and higher heating costs. So I would think that unless you tighten up your envelope the higher temp may cost you more.

Reset is more effective with hot water heat and personally I think it is more about comfort and if there are any savings that is a bonus.

joel_23

ducts

lots of cold air drops out of the ducts. Even in a well insulated duct sytem you get convective currents inside the duct system. My A/C only sytem is off for the winter .I close all the dampers. With them open I measured 45f air at -5f outside. This is a well sealed system as it is my own freakin house! I don't know if you can blame the HVAC guy for this totaly though . As an HVAC contractor I argue strongly for what I said above. No biulder or architect wants to hear it. I also argue for at least letting me put the airhandler in a closet upstairs as opposed to the attic. Again nobody wants to hear it. But then when we get problems like you've got they only call me. Everybody thinks it's an HVAC problem when more often than not it's an envelope problem.
Outdoor reset works by lowering boiler water temps. For every 3f you lower the water temps you save 1%. it works very well in my own house after installing outdoor rest my costs went down and comfort went up. I have a panel radiation system.
However in your case I can see that it might actually hurt you because extended run times will exasperate the poor duct sytem which may offset the gain reset can give. I don't know of any good formula to prove it one way or the other.
The reset is not the route of the problem though. We reset hydro air systems all the time and they work geat , but we're paying attention to all those other duct details first.
How are the boilers controlled are they step fired? you mention you have two of them. If so that's where reset also plays a big role. It's much better to step fire two boilers than fire up one big one.
Besides sealing/insulating duct sizing is important as well. esp return ducts . Do you have one central return or a return in each bedroom?. Lack of returns can presurise/depresurise different rooms. This would also be worse with longer run times. Again that's a duct problem not a reset problem ,reset might just make it more apparent.

Constantin

Hey Joel...

I guess it was a rather lame attempt to poke fun. If I offended you, please accept my apologies... I really didn't mean to grandstand, take credit, or whatever as that is not my style.

S. Orbine

Thanks again for all the thoughtful replies.

To answer your question, there are two Burnhan RV5, conrolled with the Tekmar 261 which stages the boilers and includes the outdoor reset. I'll also hint here at the state of the trade in central NJ: Instead of using a tekmar 262 (which includes a priority zone for domestic hot water) he used a the 261 with DHW as one zone. I learned this the first hot summer day when I had no hot water!

hydronicsmike

Steve

If you haven't figured it out yourself already, I would be happy to walk you through the wiring of the DHW Tank to the 261. Call 250-545-7749, Ext. 214 if you like to do that.

Regards,

Mike

John Cockerill

They are better than an aquastat constantly set at 190. You can do it all from the thermostat now. Are we heating the outdoors?

John Cockerill Exquisite Heat
www.Exqheat.com

John Cockerill

Do it yourselfe. Do time studies with different aquastat settings, fire times, while leaving the thermostat at the same setting. I have forms I used in my initial research, if youd like.


John Cockerill Exquisite Heat
www.Exqheat.com

John Cockerill

Your study sounds like something was missing.

John Cockerill Exquisite Heat
www.Exqheat.com

John Cockerill

The boiler fire is going to burn you fingers no matter what the aquastat is set at. The temperature range of the water or steam pressure ids a result of total fire BTU across the wall. That wall is very hot, exactly as hot coming off the nozzle at what ever. By the time the fire goes out the wall is very hot and has little effect. There is a little glass on the boiler face you can watch the flame through. As you change the aquastat the flame is the same. Now as far as getting the nmost out of that flame, you can do much better with a circulating warm water than you will with a stagnant boiling pot. Again the increased circulation at lower temperatures gets the BTU's out of the boiler/furnace into the house. That is the goal.

John Cockerill Exquisite Heat
www.Exqheat.com

John Cockerill

For hydronic systems you get 1 % for every 2 degrees reduction of boiler temp. You are saving it by not heating it up in the first place, by not leaving the maximum heat in the boiler and pipes when the thermostat satisfies, and by circulating those BTu's evenly to all your radiating surfaces, allowing the thermostat to call more accurate demand. It really is amazing when you get the perfect temperature adjustment every hour automaticaly. The savings are unpublishable. No one will believe you. Try it, keep charts. Don't tell the family. They will tell you how comfortable the house is by staying away from the thermostat and dressing differently. Less dry nose, dry skin, and all that jazz.

John Cockerill Exquisite Heat
www.Exqheat.com

John Cockerill

And why not Arostotle say the most wisdom comes from dialogue. When you combine the experience and thinking capabilities of many brains looking for the truth the power of numbers is awesome. Welcome to democracy.

John Cockerill Exquisite Heat
www.Exqheat.com

billygoat22

Another reason to put insulation in rafters rather than cieling- continuity of the insulation.
In a building there's a lot more penetrations in ceiling as opposed to roof deck. As changes are made to building holes are added to cieling insulation, but rafter insulation wound be unlikely to be harmed- or stepped through.
Worst insulation idea ever? Staple 6" batts in bottom chords of 2x4 trusses over suspended cieling- if they don't fall out by themselves they'll fall when someone walks in attic. Even with that setup intact the stack effect must be like a screen door for a cieling.

S Ebels

Calculating heat loss in duct.................

This is a rough way of doing it but it will get you in the ballpark.

Measure the air temp in the plenum or right at the begining of the duct. Take it at several spots and average them. Say for example it's 130*.

Now measure the air temp at the outlet of the last register on your supply duct or the end of the supply duct itself. The difference is the number of degrees you are losing as the air travels along the duct. Remember though that some of the air closer to the plenum is actually leaving at a higher temp.

Lastly, look up the CFM your blower is putting out. This too, is an approximate unles you know the static pressure you're dealing with. Let's just say you have a 3 ton blower that's rated at 1,200 CFM.

Now here's your formula. Temp difference X CFM X 1.08 = BTU's so let's say that you have a 9* temp drop in the air for the sake of doing the math.

9 x 1,200 = 10,800 , 10,800 x 1.08 = 11,664 btu's lost in this particular example.

If you want to get real technical about it you'd have to measure the temp drop and airflow at each outlet and average them to see what's really going on.

Hope this helps.

joel_19

offended

Hey Constantine! huh did it sound that way? I wasn't offended at all , thought you might be cause i didn't give you credit?

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Constantin

No, I wasn't offended either...

... hence, the smiley-face and all that. I love the wall, it's all about the learning! However, since we just write and never see the faces of the people we're writing to, it's always good to ensure that they're just as amused, happy, etc. as you are. Hence, my note. All the best, CvW